Disintegration of Long-Period Comet C/2019 Y4 (ATLAS): I. Hubble Space Telescope Observations
Quanzhi Ye, David Jewitt, Man-To Hui, Qicheng Zhang, Jessica Agarwal, Michael S. P. Kelley, Yoonyoung Kim, Jing Li, Tim Lister, Max Mutchler, Harold A. Weaver

    "Near-Sun Comet C/2019 Y4 (ATLAS) is the first member of a long-period comet group observed to disintegrate well before perihelion. Here we present our investigation into this disintegration event using images obtained in a 3-day  Hubble Space Telescope campaign. We identify two fragment clusters produced by the initial disintegration event, corresponding to fragments C/2019 Y4-A and C/2019 Y4-B identified in ground-based data...The cause of the initial fragmentation is undetermined by the limited evidence but crudely compatible with either the spin-up disruption of the nucleus or runaway sublimation of sub-surface supervolatile ices, either of which would lead to the release of a large amount of gas as inferred from the significant bluing of the comet observed shortly before the disintegration. Gas can only be produced by the sublimation of volatile ices, which must have survived at least one perihelion passage at a perihelion distance of q=0.25~au..." https://arxiv.org/abs/2105.02269

"Comet C/2019 Y4 (ATLAS) was discovered by the Asteroid Terrestrial-impact Last Alert System ( ATLAS; Tonry et al. 2018 ) program on 2019 December 28 and was immediately noted by M. Meyer for an orbit that closely resembles another LPC, C/1844 Y1 (Great Comet). 11 Further investigation by Hui & Ye (2020) supports the idea that the two are the products of a larger comet that likely split during its last perihelion passage ∼5 kyr ago. The comet pair shares a perihelion distance of q = 0.25 au and an inclination of i = 45°." https://iopscience.iop.org/article/10.3847/1538-3881/abfec3
--------------------------------------------------------

After chasing in March 2020 the Comet C 2017 T2 PANSTARRS, Maurizio next target was this comet in April 2020 were he did fourth sessions on it. The best session even with short exposure is the final image you see, but for comparison there is the other versions posted witch difference is the number of frames, time exposure and ISO setting used.

Session 2020-04-04 / Version 'Original'
54 x 45' ISO 1600
All frames were used. One stack with all frames was done for the base stars then worked the starless version with StarNetV2. The frames were split in two groups of 22 impair for stacked the comet with 1.65 sigma rejection. Then combined both parts in Ps.  Noisy background and remanent streaked stars were fixed in CamRaw and AIDenoise. Enhance comet with CamRaw.

Session 2020-04-06 / Version B
75 x 45' ISO 1600
72 of 75 frames were used. One stack done for the base stars. Worked the starless version with StarNetV2. Frames were split in two groups (36 each impair) for stacked the comet with 1.7 and 1.65 sigma rejection. Combined both part in Ps.The noisy background and streaked stars noticed were fixed with CamRaw and AIDenoise.  Enhance comet worked in CamRaw.

Session 2020-04-16 / Version 'Final'
302 x 30' ISO 2500
Some of the frames of this session presented clouds cover so in the end only 276 of the 302 frames were used for all the workflow. Same as above frames were split in two groups: 138 impair frames for work the stack comet part.  I did not need split the frames in three groups as the result of the two stacks were very nice and with really less background noise and streaked stars. Combined both parts in Ps, fixed the noise with AIDenoise and enhance the comet in CamRaw.

The result image was the best of all sessions  that could catch more the details of the comet and the reason for showed as 'Final'

Session 2020-04-22 / Version D
31 x 60' ISO 1600
All the frames were used. This was the shortest session and the result is poor in details and difused comet as you can notice.  Just for the record I process it for have a comparison between all the other sessions. Same workflow done as all the others.

Version E
I did a stack of the Astrobin Sky Plot for to see the comet route done in those sessions

Note:  this peculiar comet, considered  can belong to long-period-comet (LPC) suffers during April 2020 a noted nucleus fragmentation that our scope was not able to catch.

Many thanks for visit us and see this work.

Processed June 2022

https://twitter.com/AstroOtus/status/1539598719355600897